Alloy formation and composition partitioning of plasmonic-magnetic Au−Fe nanoparticles embedded in sol-gel SiO2 films

A gradual compositional evolution has been observed in bimetallic Au−Fe nanoparticles (nps) incorporated in SiO2 films fabricated on glass substrate by the sol-gel technique. UV–vis spectra of these nps embedded films showed a red-shifting of the Au surface plasmon resonance (Au-SPR) band after heat-treatment in reducing gas (H2−Ar) atmosphere. GIXRD and TEM studies revealed the formation of Au−Fe alloy nps (average Fe content ~21.4 at%) after reduction at 800 °C in H2−Ar. At this stage a major fraction of added Fe remains in the amorphous film as −Si−O−Fe− network. Further reduction at 900 °C results in crystallization of silica film into cristobalite with expulsion of Fe from the network. The released metallic Fe got dissolved in the existing alloy nps, and formed Au−Fe alloy with relatively higher average Fe-content (~36 at%). A detailed TEM study of the film reduced at 900 °C using nano-probe electron beam STEM−EDS technique revealed size dependent compositional change in individual alloy nps. Existence of multiple sets of fcc, and bcc peaks in GIXRD, and XPS analysis confirmed such compositional changes in the nps. The 900 °C-annealed film showed room temperature soft magnetic behavior with modified plasmonic feature confirming the existence of plasmonic-magnetic dual properties.